Differential current mode interfaces are currently used in computer systems to connect storage devices, such as disks and tapes, to intelligent host interfaces, such as storage controllers. Such differential current mode interfaces generally consist of a plurality of pairs of wires running from the storage controller with each such pair being connected to a number of storage devices in a "daisy chain" configuration. Each such pair of wires is terminated in its differential, characteristic impedance at both ends. Drivers and receivers used with these interfaces are designed such that the impedance they present to the interface is much higher than that of the termination so that they do not substantially influence the overall interface impedance as seen from any point on the interface.
Ideally, a driver for these interfaces should be a perfect current source from which perfectly equal sink and source currents can be steered to respective ones of the two lines in each pair of wires. In theory, a simple network of linked switches that are selected by the polarity of the data input applied to the driver to cause a differential current to flow through the terminating resistors and create a differential voltage that can be sensed by any receiver on the interface will accomplish this result.
However, for reasons not pertinent to the present invention, present common-mode systems, are not and cannot be well terminated. Because of the poor terminations, the interface becomes a radiator of common-mode noise which is often converted to differential mode noise when coupled into the differential interfaces. Any imbalance that occurs in the driver current results in a common-mode component being transmitted onto the interfaces causing common-mode noise to radiate from the interfaces which, in turn, results in data transmission errors.
The driver circuits now presently used with such interfaces are based in the use of NPN bipolar transistors. These circuits work well for the differential mode but have a problem going from the inhibited (zero current output) to the enabled mode for they generate excessive common-mode noise due to the large voltage swing required at several nodes in the current source part of the circuit. This voltage swing induces a delay not matched in the current sinking part of the circuit. This induced delay causes the current sinking part of the circuit to become established before the current source part of the circuit is fully activated resulting in a negative common-mode noise transient on the interface. With the prior art circuits, the only solution, to resolve this problem in the driver circuit, was to degrade the performance of the interface to limit the conditions that would generate the common-mode noise.
Accordingly, to utilize the full performance capabilities of the interface, there now exists a need for an improved driver circuit arrangement which avoids all the above described problems associated with the excessive generation of the common-mode noise in such interfaces.
The present invention, achieves these desirable results, by bringing the data inputs differentially into the circuit to assure that the current source part of the circuit is fully activated either simultaneously with or before the current sinking part of the circuit.